the cs_user_head_losses function is used to compute the values of the head loss term, and is called at each time step for each previously defined head loss volume zone.
(cs_user_zones.c and cs_user_head_losses.c)
Volume zones may be defined using the GUI, or through the cs_user_zones (in cs_user_zones.c).
cku is the local head loss term.
It appears on the momentum as follows:
![\[ \rho \der{\vect{u}}{t} = - \grad p + \vect{headloss} \: (+\: \text{other terms})\]](form_652.png) 
with
![\[ \vect{headloss} = - \rho \tens{cku}\cdot \vect{u} \,\: (\text{in } kg\cdot m^{-2} \cdot s^{-1})\]](form_653.png) 
For a distributed head loss, let  given by the litterature (
 given by the litterature (  is the head loss per unit length)
 is the head loss per unit length)
the source term tspdc is equal to 
we have 
For a singular head loss, let  given by the litterature (
 given by the litterature (  is the singular head loss)
 is the singular head loss)
the source term tspdc is equal to 
![\[\frac{\tens{dh_s}}{L} = - \frac{\tens{\xi_l}}{L} (0.5 \rho\vect{u}^2)\]](form_660.png) 
. We have
![\[\tens{cku} = 0.5\frac{\tens{\xi_s}}{L}|\vect{u}|\]](form_661.png) 
where  is the length over which we have chosen to represent the singular head loss.
 is the length over which we have chosen to represent the singular head loss.
Here is the list of examples: